First, let us consider a sled of mass being pulled by a constant, horizontal for

Question

First, let us consider a sled of mass being pulled by a constant, horizontal force of magnitude along a rough, horizontal surface. The sled is speeding up.
Part A
How many forces are acting on the sled?
one
two
three
four
Correct


Part B
The work done on the sled by the force of gravity is __________.
zero
negative
positive


Part C
The work done on the sled by the normal force is __________.
zero
negative
positive


Part D
The work done on the sled by the pulling force is __________.
zero
negative
positive


Part E
The work done on the sled by the force of friction is __________.
zero
negative
positive

Explanation / Answer

Hello !! PART A: The forces are: the weight, the normal force (the force of the ground on the sled), the frictional force and the horizontal force ==> FOUR PART B: The work done on the sled by the force of gravity is ZERO because there is no change in the gravitational potential energy (altitude does not change => the sled does moves up or down but horizontally). Another explanation is: The force of gravity is vertical which means it is perpendicular to the motion. The work of gravity is equal to distance*magnitude of gravitational force* cos(angle between direction of gravitational force and direction of distance) => BUT the angle between direction of gravitational force and direction of distance is 90º so cos of 90º is 0. Which makes the work equal to 0. PART C: The work done on the sled by the normal force is ZERO: The normal force is vertical which means it is perpendicular to the motion. The work of normal force is equal to distance*magnitude of normal force* cos(angle between direction of normal force and direction of distance) => BUT the angle between direction of normal force and direction of distance is 90º so cos of 90º is 0. Which makes the work equal to 0. PART D: The work done on the sled by the pulling force is POSITIVE because the pulling force is in the direction of the motion. In other words, it enables the motion/ it is not preventing the sled from moving but rather making it move. PART E: The work done on the sled by the force of friction is NEGATIVE because it is preventing the sled from moving / opposed to the motion. I REALLY hope this helps :) Have a great day!! :)

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